Fix a bug that was causing rake to overwrite SHELL settings
Fix a bug that was causing rake to overwrite SHELL settings

The prezto install procedure now checks to make sure the user
doesn't have an alternative version of zsh installed before changing
the current shell preference with "chsh".
This may be the case when an user is already using zsh but on a
different version from the one shipped with OS X

Fixes #264

drajguru/dotfiles,svdberg/dotfiles,stinoga/dotfiles-1,gnpkrish/yadr,nikolai-b/dotfiles,fayimora/dotfiles-1,daxgames/dotfiles,pulni/mac_dotfiles,ds82/dotfiles,nyxwulf/dotfiles,fayimora/vps_dotfiles,ds82/dotfiles,kierangraham/dotfiles,minhoolee/skwp-dotfiles,ernestonakamura/dotfiles,ChrisMcKenzie/dotfiles,sayar/dotfiles,bobek/dotfiles,wesleyskap/dotfiles-1,tmullen/dotfiles,argami/dotfiles,walkleyn/dotfiles,kylewest/yadr,atsapov/dotfiles,gizero/dotfiles,robinrob/framework,phongjalvn/dotfiles,sayar/dotfiles,fernandoaleman/yadr,jckdrpr/dotfiles-1,fogisland/dotfiles,olslash/dotfiles,ds82/dotfiles,tallamjr/yadr-fork,333fred/dotfiles_yadr,tonynguyen68/dotfiles,alex-wood/dotfiles,KonradJanica/yadr-tm,uni380/dotfiles,minhoolee/skwp-dotfiles,cantoraz/yadr,pairshaped/dotfiles,nrolland/dotfiles,j-mcnally/dotfiles,goorockey/dotfiles,hophacker/dotfiles,jf87/dotfiles-yadr,trongrg/dotfiles,plribeiro3000/dotfiles,hunterowens/dotfiles-2,daxgames/dotfiles,mente/dotfiles,tosbourn/dotfiles,andreazevedo/mac_dotfiles,sangtt/mdotfiles,mente/dotfiles,tyok/dotfiles,uhjish/dotfiles,tripl3inf/dotfiles,andrewSC/dotfiles,eywalker/walkspace,neilor/dotfiles,xkonni/dotfiles,hderms/dotfiles,sideci-sample/sideci-sample-dotfiles,aqibmushtaq/dotfiles,kierangraham/dotfiles,swinstanley/dotfiles-1,summera/yadr-alt,jckdrpr/dotfiles-1,zhang-ning/yadr,skjerven/dotfiles,argyakrivos/dotfiles,valentinzberea/dotfiles-yadr,negativo/dotfiles-2,tkuichooseyou/dotfiles,uhjish/dotfiles,neilor/dotfiles,333fred/dotfiles_yadr,goorockey/dotfiles,Jojo-Jojovich/dotfiles,jameszaghini/dotfiles,tigershen23/dotfiles,martco/yadr,zelga/dotfiles,rrmartins/my_yadr,patarra/dotfiles,neanias/dotfiles,chenhoward/dotfiles,kqyang/dotfiles,rhysyngsun/dotfiles,eywalker/walkspace,rhysyngsun/dotfiles,joeyyang/yadr,phongjalvn/dotfiles,aqibmushtaq/dotfiles,c-j-j/dotfiles-yadr,katgironpe/dotfiles,swanandp/dotfiles,sandeepjain/dotfiles,neanias/dotfiles,mrap/dotfiles,KonradJanica/yadr-tm,joesus/dotfiles,drajguru/dotfiles,mrap/dotfiles,mdarnall/dotfiles,tallamjr/yadr-fork,ptgamr/dotfiles-2,patarra/dotfiles,uhjish/dotfiles,captainmike/dotfiles,uhjish/dotfiles,backus/yadr,demis001/dotfiles,talbright/skwp-dotfiles,aeikenberry/yardr_dotfiles,cfabianski/yadr-dotfiles,bob2314/dotfiles-1,nkgm/dotfiles,fayimora/vps_dotfiles,rockwarrior/dotfiles,NikitaAvvakumov/dotfiles,marcvreuls/dotfiles,ernestonakamura/dotfiles,fabiopelosin/dotfiles,raiseandfall/vim-tmux-dotfiles,lhrkkk/dotfiles_old_2015_11_01,tyok/dotfiles,K3yChain/yadr,tonynguyen68/dotfiles,andrewSC/dotfiles,333fred/dotfiles_yadr,kanche/dotfiles,btroo/dotfiles,tyok/dotfiles,jo-tez/yadr,ttriggs/dotfiles,stevenbarragan/dotfiles,rrmartins/my_yadr,juvoni/dotfiles-1,josanastrid/dotfiles,ramiroaraujo/dotfiles,Witos/dotfiles,kdheepak89/yadr-dotfiles,lhrkkk/yadr,denyago/dotfiles,Xiaobin0860/dotfiles,agamble/dotfiles,andrewSC/dotfiles,ofer987/dotfiles,marcvreuls/dotfiles,ashlynbaum/dotfiles,nrolland/dotfiles,juvoni/dotfiles-1,fayimora/dotfiles-1,tullie/yadr-tm,hulajesus/dotfiles,nikolai-b/dotfiles,fayimora/vps_dotfiles,SherrySeibel/dotfiles,eesdil/yadr,kerbyfc/kdr,K3yChain/yadr,davidjaw/dotfiles,pruan-rht/dotfiles,pulni/mac_dotfiles,mrap/linux-dotfiles,1gitGrey/dotfiles-1,dsimmons/dotfiles-old,ay65535/yadr,akitaonrails/dotfiles,mente/dotfiles,hderms/dotfiles,fayimora/dotfiles,gizero/dotfiles,qianthinking/dotfiles,periodic1236/dotfiles,skwp/dotfiles,wesson-yi/dotfiles,uhjish/dotfiles,xytis/dotfiles,cfabianski/yadr-dotfiles,mba811/dotfiles,c-j-j/dotfiles-yadr,eesdil/yadr,josanastrid/dotfiles,hulajesus/dotfiles,demis001/dotfiles,ashlynbaum/dotfiles,RyanMacG/dotfiles-1,Sumukh/dotfiles,xvaara/dotfiles,fayimora/dotfiles-1,xytis/dotfiles,davepgreene/yadr,xytis/dotfiles,ChrisMcKenzie/dotfiles,rbrcurtis/yadr,phongjalvn/dotfiles,epelz/dotfiles,hunterowens/dotfiles-2,ramiroaraujo/dotfiles,codedecay/dotfiles-1,fayimora/dotfiles,lyanderson/dotfiles,Sumukh/dotfiles,Xiaobin0860/dotfiles,uhjish/dotfiles,trongrg/dotfiles,bionix/dotfiles-1,NikitaAvvakumov/dotfiles,jby/dotfiles,fongoses/dotfiles,GiorgioNatili/vim,andreazevedo/mac_dotfiles,sevenadrian/dotfiles,leiklier/dotfiles,koheku/dotfiles,pswaminathan/dotfiles,uhjish/dotfiles,lhrkkk/dotfiles_old_2015_11_01,yjlintw/dotfiles,davepgreene/yadr,lfilho/dotfiles,raiseandfall/vim-tmux-dotfiles,qianthinking/dotfiles,alex-wood/dotfiles,yjlintw/dotfiles,swanandp/dotfiles,agamble/dotfiles,davidjaw/dotfiles,LouTheBrew/dotfiles,xkonni/dotfiles,theone4ever/dotfiles,rbrcurtis/yadr,uhjish/dotfiles,fabiopelosin/dotfiles,gupta-tarun/dotfiles,ohnorobo/dotfiles,ptgamr/dotfiles-2,minhoolee/skwp-dotfiles,ktemkin/dotfiles,captainmike/dotfiles,sztupy/dotfiles,epelz/dotfiles,SherrySeibel/dotfiles,diego9627/dotfiles,nakolkin/dotfiles,uni380/dotfiles,jf87/dotfiles-yadr,pswaminathan/dotfiles,sztupy/dotfiles,periodic1236/dotfiles,kylewest/yadr,giorni/dotfiles,leiklier/dotfiles,brazencavalier/dotfiles,chenhoward/dotfiles,kqyang/dotfiles,pairshaped/dotfiles,btroo/dotfiles,kdheepak89/yadr-dotfiles,sangtt/mdotfiles,xvaara/dotfiles,valentinzberea/dotf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